This invention relates to solid, low impedance glass electrode structures for use in radio capsules used in the remote determination of ion activities.
The use of glass electrode structures for determining and measuring an ionic species in ionic solution is now common-place. In one usual form, the glass electrode structure comprises a tube made, at least in part, of an ion-sensitive glass. Glasses preferentially sensitive to hydrogen ion, potassium ion, sodium ion and the like are well known. The tube is sealed at one end to form an enclosure in which is disposed an electrolyte. The electrode structure also includes a conductive lead wire in contact with the electrolyte for picking up any potential at the electrolyte. When the tube exterior is in contact with a solution containing ions to which it is sensitive, a charge developes across the glass between the external solution and internal electrolyte in accordance with the ionic activity. The glass electrode structure in contact with the solution essentially constitutes a half cell.
The previously described glass electrode structure is normally employed with conventional pH equipment, the active ion sensing electrode structure being operatively connected to a standard half cell by means of high impedance, high gain electrometric amplification equipment. Such high impedance equipment is required to measure the potential of the ion sensing electrode structure since the resistivity of ion-sensitive glasses is about 10.sup.10 ohm-cm, even for so-called low resistance glasses which exhibit substantial ion sensitivity. Hence, the usual practice is to form the membrane of ion sensitive glass as a thin bulbous structure to reduce the impedance thereof. Even these electrodes may have an impedance of 100 megohms or more due to practical size limitations of the exposed sensing area.
While such electrode structures measure ion activity fairly rapidly and accurately, and are well adapted for continuous measurement, their usage poses several problems. Due to their inherent fragility arising out of the necessary thinness of the membrane, such electrodes are subject to breakage and failure. Furthermore, when attempts have been made to miniaturize this type of electrode the impedance thereof increases as the area of the ion sensitive membrane decreases, thereby placing an additional burden on the electrode and lead insulation and on the electrometric amplification equipment which must possess extremely high input impedance.
Attempts to provide stronger electrodes have resulted in relatively high impedance structures. U.S. Pat. No. 3,282,817 issued Nov. 1, 1966 to J. H. Riseman et al. discloses an ion-sensitive glass tube in which there is disposed the usual metallic, electrically conductive lead wire and a mass of solid, electrically conductive fused crystalline material in contact with both the lead wire and the ion-sensitive glass. This crystalline material may consist of a salt of silver or thallium. Electrodes of this type made with fused silver chloride, silver bromide, silver iodide or mixtures thereof are generally of high impedance, e.g., 100 to 1,000 megohms, the glass membrane accounting for only a small part of the total impedance. It appears that a very thin film of silver salt offers a high resistance in such a dry electrode structure and extremely small electrodes of this type cannot be used in application where low impedance is required.
In recent years attempts have been made to incorporate small ion-sensitive electrode structures in radio capsules small enough to be swallowed by a patient. Such electrode structures must be rugged and must provide a relatively low impedance output. Many of the techniques employed in normal laboratory monitoring of ion activities cannot be applied to radio capsules due to the small size thereof. The impedance of the aforementioned silver chloride filled electrode structure is too high to be conveniently used in a radio capsule. When well-known techniques have been applied in the development of radio capsule electrodes, inaccurate devices have resulted due to the simplicity of the circuitry, the low voltage and power available, and the requirement to insulate adequately in the small volume available in radio capsules. Compromises have been made by using low impedance metal-metal oxide or other types of pH sensors to simplify the circuitry problem. For example, U.S. Pat. No. 3,133,537 issued May 19, 1964 to H. Muth and U.S. Pat. No. 3,340,866 issued Sept. 12, 1967 to H. G. Noller disclose low impedance antimony electrodes used in conjunction with a pH measuring radio capsule. Although such electrodes are rugged and provide a low impedance output, they do not provide the accuracy which can be obtained from glass electrode structures.
Glass electrode structures having very thin liquid filled glass membranes have been considered for use in radio capsules, but the thin membrane is very easily broken during ordinary handling and represents a hazard to a patient who has swallowed such a capsule. Baskets made of corrosion resistant metal have been employed on some of these capsules to protect the fragile, ionsensing glass membrane. The protective basket restricts the flow of fluid over the membrane, thereby causing a response time lag and giving rise to the possibility that matter will be trapped by the basket and interfere with access of the electrode surface to the fluid under test.